Planar high-voltage semiconductor rectifier and switching devices typically require edge termination around the high-voltage terminal pn junction. In general the goal is to produce a smoothly tapered field profile to prevent high-voltage field-crowding that causes premature breakdown at the edge of the high voltage electrode. Without such a termination, the regions of high field curvature around the edges of the terminal reach the fundamental breakdown voltage far below the parallel plane limit. Such premature breakdown can be reduced somewhat by means of a field-plate or single-zone junction termination extension (JTE), but maximum performance is typically achieved through a more diffuse field profile in which the carrier concentration decreases with the distance from the high-voltage anode or cathode.
High-voltage junction terminations are commonly formed by creating a series of zones of progressively lower doping around the high-voltage anode or cathode. The doping is introduced by ion-implantation or diffusion through a blocking mask. In silicon, a highly effective planar termination technique involves use of a complex checkerboard implantation mask through which fine-scale islands of dopants are implanted into the silicon base. By reducing the ratio of the island size to the unimplanted area, a graded termination doping is achieved after diffusion. The surface field reduction achieved by this method is ideal since the doping is smoothly varying across the termination, and by specifying the mask design and implantation process, detailed control over the width and shape of the resulting doping profile in the edge termination can be achieved.
Silicon carbide (SiC) has become an increasingly popular semiconductor material for high-voltage devices due to its high breakdown electrical field and its relative insensitivity to high temperatures. However, dopants do not diffuse effectively in silicon carbide, and thus the dopant island-implantation technique that provides a smoothly graded junction termination doping profile in silicon does not work well.
Instead, in silicon carbide, fabrication of graded edge termination in high-voltage rectifiers and switches has generally been accomplished by using multi-zone or by floating guard ring methods. Both of these methods for producing graded junction termination extensions can result in excellent parallel-plate breakdown voltage, but both also have significant drawbacks.
In the multi-zone method, multiple zones of material having decreasing dopant levels are placed around the anode/cathode at the center of the device. The doped zones are electrically connected and overlap to provide a continuously stepped doping profile from the center of the device to the edge. However, fabrication of a graded junction termination extension using this method requires multiple doping steps, and therefore can be expensive, both in terms of time and money. In addition, each transition between the stepped regions is a potential breakdown point due to differences in doping levels and differences in voltage in each zone, and at high voltages (i.e., higher than 2.4 kV), these differences can result in a complete breakdown of the device.
The floating guard ring method utilizes a tightly graded series of micron-scale doped rings to create an approximation of a continuous doping profile. Although this method can produce a graded junction termination in one doping step, it requires the use of a very precise doping mask and high-resolution photolithography, and therefore can be even more expensive than the multi-zone method.
Thus, the established techniques for producing a graded junction termination extension can be expensive and/or complicated. Furthermore, performance of these techniques often suffers because of the existence of high electric fields in the transition between doping zones. Simpler, less costly, and more effective methods are widely sought, but to date these solutions are limited in the lateral width of the termination region, are limited in the lateral field profile that can be achieved across the region, and/or are limited in the tone of the photoresist that can be used.